This project proposes a neurophysiological and neuropharmacological study of the caudate nucleus in a model of Parkinson's disease produced by the interruption of the nigrostriatal pathway in cats. We wish to test the hypothesis that the normal functioning of the caudate nucleus depends on a balance between cholinergic, dopaminergic, GABAergic, and serotoninergic action and that Parkinson's disease is due to a disturbance of this balance. To this end, we will compare the effects of acetylcholine, dopamine, GABA and serotonin with the effects of stimulating structures giving rise to afferent fiber tracts to this nucleus. Using antagonists, we will attempt to identify the actions of these agents with the effects of stimulating the individual synaptic inputs. These experiments will be performed in cats with and without nigrostriatal lesions in order to demonstrate changes in sensitivity to one or more imputs or transmitter candidates. We will use multibarreled micropipettes to apply drugs microiontophoretically near single neurons in the caudate nucleus and to record synaptic responses. These responses will be enhanced by the computation of poststimulus histograms and of average focal evoked potentials.